228 a. T. JACKSON — studies of pal.eechinoidea. 



whether or not the arrangement is exactly as in plate 7, figure 42, but 

 should say that it was essentially the same. In one of these specimens, 

 from Licking county, Ohio, which is preserved entire, the ambulacra are 

 much broader on the ventral aspect and are decidedly narrower near the 

 dorsal area. The other specimen,* which is from Meadville, Pennsyl- 

 vania, is flattened on the rock and represents the dorsal portion of the 

 corona. The plates on the lower or ventral portion of this specimen are 

 rounded and imbricating, but near the dorsal portion they are hexagonal 

 and less imbricate, and close to the dorsal termination of the areas are 

 nearly, and in some cases perhaps actually, rhombic in form, closely re- 

 sembling the plates of the same region in Melonites (plate 3, figure 13) ; 

 they are very small and need to be studied critically. 



This rhombic and hexagonal form of the dorsal or newly added plates 

 of Lepidechinus rarispinus, it seems, is an important character, for in it we 

 have newly added or, so to speak, young plates which have characters seen 

 normally in less specialized genera. The peculiar scale-like and imbricat- 

 ing character appears later, when the plate has been considerably removed 

 from the dorsal area by the intercalation of later added plates. An im- 

 portant fact bearing directly on this problem is Mr Agassiz's observation 

 (Challenger Echini, page 75), that 3''0ung Phormosoma and Asthenosomaf 

 show only the slightest possible lapping of the edges of the plates of the 

 corona, and it is only in somewhat later stages that the lapping ot the 

 sutures becomes apparent. This change in the form of plates in Lepi- 

 dechinus reminds one strongly in the im[)lied principles of growth of the 

 change in form of plates of some crinoid stems. In modern Pentacrinus 

 and Metacrinus the plates of the older part of the stem are nearly round ; 

 but the plates close up under the calyx, which are the new, last added 

 plates, are strongly pentagonal, resembling closely the form of plates seen 

 throughout the length of the stem in the Jurassic genus Extracrinus. We 

 have in this structural development of the crinoid stem apparently a direct 

 phylogenetic feature, in which the young, newly added plates throughout 

 the life of the individual resemble the plates of the entire stem in ancient 

 representative types. It is a curious and important feature of stages in 

 growth — a sort of continual rejuvenation in a localized part — which has 

 not, so far as known, been made use of in phylogenetic studies. In Lepi- 

 dechinus rarispinus I would venture the suggestion that the rhombic and 

 hexagonal plates of the dorsal area in a measure have phylogenetic sig- 

 nificance and indicate that the species was derived from forms which did 

 not have imbricating plates. This is in accordance with what we might 



*Thi.s specimen is tlie original of plate 9, figure 10, of the Twentieth Report, New York State 

 Cabinet, 

 t Recent deep-sea members of the Echinothuridse, 



